Thermal actuator

ABSTRACT

An actuator with a thermally responsive elongated body of expansible material adapted for changes between solid and liquid states. A heat transfer mechanism produces incrementally varying temperatures longitudinally of the elongated body so as to provide progressive melting and hardening thereof.

United States Patent Inventors Howard A. Powers [56] References Cited MFdIiQId; UNITED STATES PATENTS n' 3,016,691 1/1962 Asakawa m1 73/3683 App] No 2 2- 2,705,270 3/1955 Moran 337/320 x Filed APL301968 2,187,258 1/1940 Wood.... 337/320(X) Patented June 1,1971 FOREIGN PATENTS Assignee Fenwal, inc. 634,852 1/1962 Canada 337/393 Ashland Mass Primary ExaminerBernard A. Gilheany Assistant Examiner-Dewitt M. Morgan THERMAL ACTUATOR Attorney-John E. Toupal 30 Claims, 6 Drawing Figs.

US. Cl 337/139,

, 337/141, 337/393 ABSTRACT: An actuator with a thermally responsive elon- Int. Cl ..l-l0lh 37/46, gated body of expansible material adapted for changes HOlh 61/06, HOlh 71/18 between solid and liquid states. A heat transfer mechanism Field of Search 337/123, produces incrementally varying temperatures longitudinally of 124, 139, 141, 306, 314, 315, 382, 393, 394, 398, the elongated body so as to provide progressive melting and 120; 338/217; 73/3683 hardening thereof.

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' I I 1 1 I I 4:3 ,6 a! is Ca (5 a -I I 50 50 I I I 46 43 F x-I I' I PATENTED JUN 1 I97! SHEET 1 OF 3 wk'r z flamme THERMAL ACTUATOR BACKGROUND OF THE INVENTION This invention relates to a thermal actuator and more particularly relates to an actuator with a partially confined body of thermally expansible material that is selectively heated or cooled to produce movement of an associated control member.

Actuators operated by thermally expansible materials have been in use for many years. They include both units responsive to variations in ambient temperature and those controlled by energizable heaters. Responsive to changes in its temperature the expansive material expands or contracts to adjust the position of a suitable control member. The present invention is directed primarily to the latter type which can be used to actuate valves, switches, dampers etc. in a wide variety of control applications.

Prior actuators have employed many types of thermally responsive materials including liquids, solids, substances, adapted for changes between solid and liquid states, and various combinations of such materials. Of these, the change of state materials exhibit particularly interesting characteristics. For example, certain synthetic waxes undergo substantial variations in volume during changes of state that occur over given relatively small temperature ranges. Accordingly, such materials can produce extensive control movements in response to limited variations in control temperature. A further advantage of some materials of this kind is that their change of state occurs at relatively high temperatures of, for example, 200 F. and above. Therefore, actuators using such materials are substantially unaffected by ambient temperature variations.

Although possessing these desirable characteristics, prior wax filled actuators also have had the undesirable feature of being comparatively slow acting. This disadvantage stems from the inherent difficulty in transferring heat into and out of the expansible wax. Since actuator response is dependent upon the temperature of the wax, heat transfer rates are obviously the primary factors affecting response time. The heat transfer problem is particularly troublesome with actuators utilizing the relatively large volumes of expansible material required to produce control movements of substantial length.

Previous attempts to improve the response time of wax actuators have been directed primarily toward increasing the rate at which the expansible wax body is heated from a solid to an expanded liquid state. Thus, various schemes have been devised for applying heat directly to the interior of the wax body. Known, for example, are embedded electrodes, internal heater elements, electrically conductive particles distributed throughout the wax body and adapted to conduct heating current therein, etc. Although reducing the time required to heat the wax from a solid to a liquid state, such innovations have done little to improve cooling times which are just as instrumental in establishing actuator response.

The object of this invention, therefore, is to provide a faster acting thermal actuator of the type employing an expansible operating medium adapted to change between the solid and liquid states within the operating temperature range of the device.

CHARACTERIZATION OF THE INVENTION The invention is characterized by the provision of a thermal actuator comprising an elongated body of expansible material adapted to undergo substantial volume variations during changes between solid and liquid states induced by changes in its temperature, longitudinal walls that restrict radial expansion of the elongated body, a piston member disposed adjacent one end of the elongated body and mounted for movement in response to volume changes therein, and a heat transfer mechanism adapted to produce longitudinally progressive melting of the elongated body. The progressive melting begins at the end of the body adjacent the piston member. The use of an elongated body provides the expansible material with a high surface to volume ratio that facilitates heat transfer into and out of the material. In addition, progressive melting prevents the buildup within the longitudinal walls of excessive forces caused by material expansion in portions of the elongated body isolated from the movable piston member by rigid, not yet melted body portions. Because excessive internal forces are prevented, relatively thin longitudinal walls can be used thereby improving heat transfer between the elongated material body and the surrounding environment.

A feature of the invention is the provision of a thermal actuator of the above type wherein the heat transfer mechanism produces longitudinally progressive hardening of the elongated body with solidification occurring finally at the end of the body adjacent the piston member. The progressive hardening prevents premature terminations of piston movement during expansible body cooling periods. This unwanted performance characteristic can result from the formation of completely solid elongated body portions that obstruct inward piston movement before other isolated elongated body portions have fully hardened.

Another feature of this invention is the provision ofa thermal actuator of the above type wherein the piston is adapted for movement through a given maximum stroke and the elongated body is capable of expanding to produce the maximum stroke before a substantial portion of the expansible material has changed from a solid to a liquid state. By providing a substantial excess of expansible material, full stroke capability is retained even after significant quantities of the expansible material have been lost by leakage during operation of the actuator.

Another feature of this invention is the provision of a thermal actuator of the above featured type wherein the heat transfer mechanism produces heat transfer into the elongated body along substantially its entire length at an incrementally varying rate that decreases progressively in an axial direction from the end of the body adjacent the piston device. The progressive heat transfer rate produces the above mentioned progressive melting of the elongated body.

Another feature of this invention is the provision of a thermal actuator of the above featured types wherein the heat transfer mechanism is adapted to dissipate heat energy externally of the elongated body at an incrementally varying rate that increases progressively in an axial direction from the end of the body adjacent the piston device, In addition to producing the desired progressive hardening of the elongated body during cooling cycles, this feature permits attainment during heating periods of the desired progressive melting even though heat is applied at uniform incremental rates.

Another feature of the invention is the provision of a thermal actuator of the above featured type wherein the heat transfer mechanism generates heat along the length of the elongated body at an incrementally varying rate that decreases progressively in an axial direction from the end adjacent the piston device. The generation of heat at progressive rates along the length of the elongated body is another preferred method for creating the desired progressive heat transfer.

Another feature of this invention is the provision of a thermal actuator of the above featured types wherein the longitudinal walls are adapted to conduct electrical current and thereby function as a heater element. Resistive heating of the longitudinal walls insures prompt melting of the expansible material contacting its inner surfaces thereby facilitating axial movement within the walls of the expanding elongated body. In addition, direct resistive heating of the walls provides this advantage without the requirement for an external heating unit that would inhibit heat transfer out of the elongated body during cooling cycles.

Another feature of this invention is the provision of a thermal actuator of the above featured type wherein the piston is adapted for movement within the longitudinal walls and is in sliding contact with the internal surfaces thereof. This arrangement reduces to a minimum the amount of expansible material between the piston and the longitudinal walls. Thus, rapid melting of this material and prompt release of the piston is insured during a resistive heating cycle.

Another feature of the invention is the provision of a thermal actuator of the above featured type wherein the piston is an elongated rod formed of an electrically nonconductive material. Elongation of the piston rod permits the use of a stationary seal assembly and thereby simplifies construction of the actuator. In addition, the use of a nonconductive material prevents the piston from shorting the adjacent section of Iongitudinal wall during heating cycles Such shorting would inhibit melting of the expansible material surrounding the piston and prevent its prompt release.

Another feature of this invention is the provision of a thermal actuator of the above featured type wherein the longitu dinal wall comprises a continuous hollow tube having one length portion wound into a coil with axially spaced turns and another straight length portion disposed within the coil portion and enclosing the piston rod. This arrangement provides in an extremely compact form the desired elongated body of expansible material and associated piston.

Another feature of this invention is the provision of a thermal actuator of the above featured type wherein the axial spacing between adjacent turns of the coiled type portion increases progressively from the end thereof joined to the straight length tube portion. The progressively increasing turn spacing provides in a simple arrangement the above described heat transfer characteristic wherein heat is externally dissipated at an incrementally varying rate that increases progressively from the end of the body adjacent the piston.

Another feature of this invention is the provision of a thermal actuator of the above featured types wherein the longitudinal wall possesses an incrementally varying electrical re sistance that decreases progressively in an axial direction from the end adjacent the piston device. The incrementally varying wall resistance provides in a simple structure the above mentioned progressive heating rates.

Another feature of this invention is the provision of a thermal actuator of the above featured type including a transducer responsive to relative movement of the piston and adapted to produce an output signal indicative of the position thereof. The transfer can be used in a control circuit to provide piston position feedback control of the actuator.

Another feature of this invention is the provision of a thermal actuator of the above featured type wherein the transducer comprises a hollow cylindrical transformer enclosed by the coiled tube portion and having a ferromagnetic core axially aligned and operatively connected to the piston rod. In this extremely compact arrangement, the transformer core functions both as a force transmission member and as a piston position indicator. Relative movement between the ferromagnetic core and cylindrical transformer alters the coupling between concentrically disposed primary secondary winding which thereby provides an output signal indicative of relative piston position.

Another feature of this invention is the provision of a thermal actuator of the above featured type including a magnetic shield disposed between the coiled tube portion and the cylindrical transformer. The magnetic shield prevents the heating current flow in the coiled turns from undesirably influencing the output of the transformer.

Another feature of this invention is the provision of a thermal actuator of the above featured type including a plurality of elongated alignment templets extending longitudinally of the coiled tube and spaced about the circumference thereof. Each templet possesses a plurality of strategically spaced openings that receive in close contact relationship the coil turns thereby establishing and maintaining the desired spacing between individual turns.

Another feature of this invention is the provision of a thermal actuator of the above featured type including a thermal detector that interrupts heating current flow in response to detection of a predetermined maximum temperature on the longitudinal walls. This feature prevents possible destruction of the actuator by excessive internal forces in the event that outward movement of the control piston is obstructed by some external condition.

Another feature of this invention is the provision of a thermal actuator of the above featured type including a coiled spring member enclosing the coiled type portion and operatively connected to the transformer core so as to provide thereon a biasing force that urges the piston rod into the straight length tube portion. In addition to forcing the core and piston rods into retracted positions during cooling cycles, the surrounding coiled spring member serves as a protective mechanical shield for the actuator.

Another feature of this invention is the provision of a thermal actuator of the above featured type including a cover supported by and enclosing the coiled spring member. The cover shields the actuator from external air currents that tend to nonuniformly alter heat transfer characteristics and thereby undesirably modify performance of the unit.

These and other objects and features of the present invention will become more apparent upon a perusal of the following specification taken in conjunction with the accompanying drawings wherein:

FIG. I is a partially broken away side view of a preferred embodiment of the invention;

FIG. 2 shows in enlarged cross section opposite end portions of the actuator illustrated in FIG. I;

FIG. 3 is a partial enlarged cross section of the middle portion of the actuator shown in FIG. I;

FIG. 4 is a cross-sectional view taken along lines 4-4 in FIG.

FIG. 5 is a cross-sectional view taken along lines 5-5 in FIG. I; and

FIG. 6 is a schematic circuit diagram illustrating a preferred control circuit for the actuator shown in FIG. 1.

Referring now to FIGS. l3,' there is shown the actuator 11 including the hollow, continuous elongated tube 12 made ofa suitable electrically conductive material such as stainless steel. The tube 12 includes the straight portion 13 and the enclosing, concentric coil portion 14 joined thereto by the connecting portion 15. Forming the coil portion 14 are the plurality of coil turns 16 spaced apart in a direction parallel to the axis of the coil 14. The elongated piston rod 17, preferably formed of a nonconductive material such as quartz, is disposed within the straight tube portion 13 in a slightly spaced relationship therewith. Filling the entire volume enclosed by the tube 12 between the piston rod 17 and the sealed end 18 is a tempera ture expansible material 19. Preferably, the material 19 comprises a suitable synthetic wax that experiences a substantial increase in volume upon changing from a solid to a liquid state in a temperature range, for example, between 215 F. and 225 F.

Surrounding and concentric with the straight tube portion 13 is the hollow shell 21 made of a suitable electrically conductive and ferromagnetic material such as low carbon steel and having the slot 22 that accommodates the connecting tube portion 15 and grommet 20. The opposite ends of the straight tube portion 13 are supported within the hollow shell 21 by, respectively the annular spacer 23 (FIG. 2) made of an electrical insulating material and the hollow cylindrical bushing 24 (FIG. 3) made of an electrically conductive material. Disposed within the cylindrical bushing 24 is the annular seal member 25 which is maintained in compression between the annular containing ring 26 and the cylindrical compression spring 27.

Also positioned within the hollow shell 21 is the transformer assembly 31 (FIGS. 2 and 3) including the rod core 32 made of a suitable ferromagnetic material such as iron or low carbon steel. Retaining the rod core 32 and providing a bearing sur face therefor is the hollow spool 33 made of a suitable nonmagnetic and low friction material such as acetal. The hollow spool 33 also functions as a form for the inner cylindrical primary winding 34 and the outer cylindrical secondary winding 35.

As shown in FIG. 3, the central recess 38 in the inner end of the transformer rod core 32 retains the insert 39 formed of a suitable resilient material such as rubber. Engaging the insert 37 is the outer end of the piston rod 17 which is axially aligned with the rod core 32. The outer end of the rod core 32 (FIG. 2) is adapted for operative connection with a device (not shown) to be controlled.

Enclosing the coiled tube portion 14 is the cylindrical spring member 43 shown in FIG. 2. The terminal coils at the ends of the spring member 43 are accommodated by annular grooves in the outer edges of the circular end plates 44 and 45. Projecting through the central portion of and attached to the outer end plate 45 is the transformer rod core 32. The inner end plate 44 possesses a central recess that receives the end of the hollow shell 21 and terminates in the bracket portion 46 adapted to facilitate mounting of the actuator 11.

Connected to the sealed end 18 of the coiled tube portion 14 is the electrical lead wire 47 that passes through the actuator before exiting through a slot in the shell 21 and a central opening 48 in the end plate 44. Also extending through opening 48 are leads 48' and 49 from the primary and secondary windings 34 and 35 and from the thermistor 49 positioned adjacent the straight tube portion 13. Enclosing the cylindrical spring member 43 is the cover 50. Preferably, the cover 50 comprises a plastic sleeve which is placed over spring member 43 and then heat shrunk into the form illustrated in FIGS. 1 and 2.

During operation of the actuator 11, the electrical lead 47 is energized to produce current flow through the entire length of the elongated tube 12. The current flows through the series circuit comprising the lead wire 47, the coiled tube portion 14, the connecting tube portion 15, the straight tube portion 13, the cylindrical bushing 24, the set screws 40, and the grounded hollow shell 21. Resistive heating generated by the current raises the temperature of the expansible material 19 to its melting point thereby causing a substantial increase in its volume. Responsive to the pressure exerted by the expanding material 19, the piston rod 17 and transformer rod core 32 are forced outward. This movement effects a control function such as activating a valve, switch, damper, etc. After deenergization of the lead wire 47, the expanded material 19 experiences a converse contraction in volume upon cooling back to the solid state. Accordingly, the compressive forces exerted by the spring member 43 force the attached transformer rod core 32 and piston rod 17 back into the actuator 11. Again, corresponding movement of the controlled member (not shown) also occurs.

As shown somewhat exaggerated in FIG. 2, the axial spacing a a ,...a, a, increases progressively between the end of the coiled tube 14 connected to the straight tube portion 13 and the sealed end 18. The variable spacing is established and maintained by the templets 50 equally spaced around the circumference of the coiled tube portion 14. Each templet 50' possesses a plurality of strategically spaced openings that receive in close contact relationship the coil turns 16. Because of this varying spacing, the elongated tube 12 dissipates heat externally at incremental rates that increase progressively between the piston 17 and the sealed end 18. Consequently, the energized tube 12 transfers heat into the wax 19 at incremental rates that decrease progressively in that direction. It will be noted that the reduced spacing a between the first two turns is exaggerated to compensatefor the fact that only one side of the first turn is adjacent another turn. Also, the thermal isolation of the straight tube portion 13 by the surrounding air space 51 insures a minimum rate of external heat dissipation in that region. For these reasons, initial melting of the wax 19 occurs in the annular space between the piston rod 17 and the adjacent wall surface of the straight tube portion 13 and subsequent melting progresses along the length of the tube 12 in a direction toward the sealed end 18.

This is an important feature of the invention as it prevents the generation of excessive internal pressures within the tube 12. Such excessive pressures could result either from having movement of the piston rod 17 restrained by unmelted wax 19 between its outer surface and the inner adjacent surfaces of the tube 12 or by the existence of melted wax pockets separated from the movable piston rod 17 by nonmelted wax portions. Either of these conditions could occur in the absence of the above described progressive melting of the wax 19 along the length of the tube 12. Because of this limitation on internal pressure, the walls of the tube 12 can be made very thin thereby enhancing their heat transfer capability.

The variable turn spacing feature also provides a desirable progressive cooling of the wax 19 upon termination ofa heating cycle. Because the elongated tube 12 dissipates heat externally at incrementally varying rates along its length, the contained wax 19 is cooled and hardened progressively from the sealed end 18 with the wax nearest the piston 17 hardening last. Therefore, inward movement of the piston rod 17 is not restrained completely until the entire body of wax 19 has become solid.

The desirable progressive heating and cooling of the wax 19 is insured further by the cover 50 that provides a controlled environment for the elongated tube 12. Without the sleeve 50, air currents caused, for example, by the chimney effect could induce changes in heat dissipation rates and thereby alter the performance of the actuator 11.

As also shown somewhat exaggerated in FIG. 2, the wall thickness 2,, t t mt t of the tube 12 increases along the coiled tubing portion 14. The variation in wall thickness is such that the cross-sectional area of the tube 12 decreases uniformly between the sealed end 18 and the straight tube portion 13 which has a constant cross section of minimum area. Consequently, the incremental electrical resistance of the tube 12 progressively increases from the sealed end 18 to a maximum value in the straight length portion 13. Therefore, current flow through the tube 12 produces incrementally vary ing heat generation rates that amplify the desired progressive melting of the wax body 19.

Another important feature of the invention is the provision ofa wax body 19 having, for example, two or three times more solid volume than required to produce a full outward stroke of the piston rod 17 when melted. This full stroke has a length substantially equal to the distance between the outer end of the connecting tube 15 and the stroke limiting piston seal 25. During each outward stroke, microscopic depressions in the surface of the piston rod 17 transport a minute quantity of wax 19 through the piston seal 25. Some of this wax is dislodged by the seal and does not return to inside of the tube 12 upon retraction of the piston rod 17. Thus, after extensive use, the total volume of solid wax l9 retained by the elongated tube 12 is decreased. However, because of the originally provided excess, the tube 12 still contains a sufficient volume of solid wax 19 to produce a full stroke of the piston rod 17. Producing a full piston stroke after wax loss merely requires that the heat input to the tube 12 be increased slightly so as to melt a quantity of wax l9 sufficient to both provide a full piston stroke and to fill the void created by wax loss.

The usefulness of the actuator 11 is significantly improved by the transformer assembly 31. Since movement of the rod core 32 changes the magneticcoupling between the primary and secondary windings 34 and 35, the signal output of the secondary winding 35 is proportional to the position of the rod 32 with respect to the transformer windings. Obviously, the transformer output signal also is indicative of the relative position of the piston rod 17. The manner in which this position indicating signal can be used in a control system is described more fully below.

The arrangement of the various individual components in the actuator 11 as shown in FIG. 1 provides an extremely compact unit. In this regard, the use of separate rods 17 and 32 is particularly advantageous although a single rod obviously could be used. The nonconductive piston rod 17 eliminates the possibility of having the straight tube portion 13 shorted during a heating cycle. Such an occurrence would greatly reduce the heating rate produced in the straight tube portion 13 and possibly prevent melting of the wax 19 located in the annular space between the piston 17 and adjacent tube wall surface. Conversely, the ferromagnetic properties of the core rod 32 induce changes in the magnetic coupling between the transformer windings 34 and 35 thereby producing an indication of piston position. Furthermore, the metal rod 32 possesses mechanical strength desirable because of its exposure during use while the transformer assembly 31 shields the weaker and more polished piston rod 17 from both corrosion and external forces.

Referring now to FIG. 6, there are shown the power input terminals 52 and 52 connected to a suitable source of AC voltage. Connected in series across the terminals 52 and 52 are the resistor 53, the diode 54, and the capacitor 55. Joining the terminal 52 and the junction 56 between the diode 54 and the capacitor 55 are the resistor 57 and the Zener diode 58 that maintains a constant DC voltage on the terminal 59. That voltage is applied to the unijunction transistor 61 that is connected in series between the resistor 62 and the primary winding 34 of the actuator 11. The emitter electrode 63 of the transistor 61 is connected to the junction between the capacitor 64 and the resistor 65 which are connected in series with the diode 66 across the input terminals 52 and 52.

The constant DC voltage on terminal 59 is applied also to the unijunction transistor 67 that is connected in series between the resistor 68 and the series combination of the adjustable resistance 69 and the thermistor 71. The emitter elec trode 72 of the transistor 67 is connected to the junction 73 between the series connected voltage dividing resistors 74 and 75 which are coupled to the constant voltage supply terminals 52 and 59. Also connected to junction 73 by the capacitor 76 is the secondary winding 35 of the actuator 11. The damping resistor 77 is connected directly across the secondary winding 35.

Connected in series across input terminals 52 and 52 are the elongated heater tube 12 of the actuator 11 and the silicon controlled rectifier 78. The control electrode 79 of the SC rectifier 78 is connected by the resistor 81 to the junction between the transistor 67 and the resistor 68. Also connected in series across input terminals 52 and 52' are the resistor 85 and the thermistor 49 that is located in the actuator 11. The control electrode 79 of the silicon controlled rectifier 78 is connected by resistor 86 to the junction between the diode 87 and the capacitor 88 which are coupled across the resistor 85. In parallel with the capacitor 88 is the diode 89.

Upon excitation of terminals 52 and 52, conduction by diode 66 during alternate half cycles of input voltage causes the transistor 61 to fire producing a current pulse in the transformer primary winding 34. The resultant induced pulse in the secondary winding 35 either charges the capacitor 76 to the firing potential of the transistor 67 or not dependent upon the magnitude of the pulse. The pulse value required is determined by the combined resistance of the adjustable resistor 69 and the thermistor 71. Naturally, the magnitude of the secondary pulse is dependent upon the degree of coupling between the primary winding 34 and secondary winding 35 which is in turn dependent upon the relative position of the core rod 32 within the transformer assembly 31. Assuming the required combination of resistance and pulse values, firing of the transistor 67 gates the SCR 78, producing current flow through the elongated tube 12. Conversely, if the transistor 67 is not fired by the secondary pulse, the SCR remains nonconductive and current flow through tube 12 is prevented.

In a typical application of the actuator 11 and the control circuit shown in FIG. 6, the operating member 32 would be connected to a device (not shown) that regulates heat flow into a particular zone. Mounted to sense the ambient temperature of that zone would be the thermistor 71. The regulating device could be, for example, the damper in a hot air supply pipe. Operation of the actuator and circuit would then proceed as follows:

The resistor 69 is manually adjusted to a resistance value corresponding to the temperature desired in the controlled zone. THen, assuming the existence of that temperature, the piston rod 17 and rod core 32 assume a given balanced control position. That position establishes a heat flow rate necessary to maintain that temperature in the controlled zone under the existing conditions of heat loss. Actually, the rods 17 and 32 oscillate slightly about the balanced position and the core rod 32 oscillations induce periodic firing of the transistor 67. This in turn produces periodic current flow through the elongated tube 12. The average value of the current flow is such as to maintain a wax volume corresponding to the balanced position of the piston 17.

Next assume an increased thermal load that reduces the temperature in the controlled zone to below the desired value. The resultant increase in the resistance of the thermistor 71 reduces the firing potential of the transistor 72 and permits continuous alternate half cycle firing thereof by induced pulses. Therefore, the frequency at which the silicon controlled rectifier 78 is gated increases producing an enlarged average current flow through the elongated tube 12. The corresponding increase in rate of heat generation produces melting and expansion of more wax and forces the rods 17 and 32 outward. Responsive to movement of rod 32 the attached heat regulator (not shown) provides greater heat flow into the temperature controlled zone. However, outward movement of transformer rod core 32 also reduces the coupling between the primary and secondary windings 34 and 35 thereby diminishing the signal value induced in the secondary winding 35. Therefore, after a certain degree of movement the induced signal value becomes insufficient to fire the transistor 67 and both gating of the SCR 78 and heating current flow through the tube 12 are interrupted. Consequently, the wax 19 begins to cool and contract allowing the spring member 43 to force the rods 17 and 32 inward. This movement quickly increases the transformer coupling until firing of the transistor 67 again occurs. Thus, the piston rod 17 and core rod 32 will resume oscillatory movement in a narrow range about a new balance position related to the resistance value of the thermistor 71. Obviously, the new balance position provides more heat flow to the controlled zone thereby tending to compensate for the increased thermal load and reestablish the desired temperature.

Finally, assuming a temperature increase in the controlled zone to above the desired value, the resistance of a thermistor 71, decreases to enlarge the potential required for firing the transistor 67. Therefore, firing ceases and heating current flow to the tube 12 is interrupted. Resultant cooling and contraction of the wax 19 continues until the core rod 32 has been forced inward into a new balance position wherein the above described oscillatory movement resumes. In this new balance position, the heat regulator (not shown) provides less heat flow to the temperature controlled zone thereby allowing a return to the desired temperature.

Thus, responsive to the ambient temperature sensed by the thermistor 71, the actuator 11 regulates heat flow to maintain in the controlled zone a given temperature range selected by adjustment of the resistor 69. In addition, the position feedback control provided by the transformer assembly 31 improves system sensitivity by preventing large overshoots in the controlled ambient temperature. Such overshoots occur with simple on-off control systems because of the inherent thermal lag of the temperature controlled zone.

Another wax filled actuator problem is eliminated in the present invention by the thermistor 49. Occasionally, the piston of an actuator is restrained from movement by some external condition. For example, either an inadvertently introduced construction of some form of damage may prevent normal movement of the device being controlled in which case the operatively connected actuator piston also is restrained. Then, with the piston unable to respond to wax expansion, the application of heat can produce internal pressures capable of bursting the thin walled tube 12. This danger is alleviated by the thermistor 49 which functions in the control circuit of FIG. 6 in the following manner. Assuming some external restraint of the piston 17, gating of the SCR 78 continues with interruption since the core rod 32 does not move to reduce transformer coupling. Consequently, the temperature of the tube 12 rises in response to the uninterrupted flow of heating current. However, at a predetermined maximum temperature the reduced resistance of the thermally responsive thermistor 49 permits a disabling level of alternate half cycle current flow through the diode 87 and the capacitor 88. This current charges the capacitor 88 with an opposition voltage that'reduces the potential applied to the control electrode 79 and prevents gating of the SCR 78. Thus, heating current flow through the tube 12 is stopped and the generation of excessive pressure therein avoided. It is possible also with the present invention to provide maximum piston stroke control. By merely winding a small outer longitudinal section 91 of secondary winding 35 in opposition to the inner section 92 a bucking secondary voltage is produced. Since initial movement of the core rod 32 reduces coupling between the primary winding 34 and only the inner section 92, this bucking voltage remains constant while the operating voltage induced in the inner section 92 is diminished. Therefore, at some predetermined piston position the relative values of the operating and bucking voltages prevent further firing of the transistor 67. Consequently, heating current flow that would cause addi tional outward piston movement is prevented.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings. For example only, the various unique features of the invention can be used independently as well as in the combination shown and described. Also, other types of heater mechanisms such as electrically conductive tube coatings or separate heater elements can be used to provide the desired progressive temperature changes in the elongated body 19. Therefore, it is to be understood that within the scope of the appended claims the invention can be practiced otherwise than as specifically described.

What we claim is:

1. A thermal actuator comprising:

a. an elongated body of expansible material, said expansible material being of a type that experiences substantial volume increase during changes between solid and liquid states and substantial volume decrease during changes between liquid and solid states, both changes in state being induced by changes in its temperature;

b. pressure responsive means disposed adjacent one end of said elongated body and movable in response to volume changes therein; and

c. heat transfer means in heat transfer relationship with said elongated body and adapted to produce incrementally varying temperatures longitudinally thereof; said heat transfer means comprising longitudinal wall means that restricts radial expansion of said elongated body.

2. A thermal actuator according to claim 1 wherein said heat transfer means comprises heating means for producing longitudinally progressive melting of said elongated body with initial melting occurring at the end thereof adjacent said pressure responsive means.

3. A thermal actuator according to claim 1 wherein said heat transfer means comprises heating means for producing heat transfer into said elongated body along substantially its entire length at an incrementally varying rate that decreases progressively in an axial direction from the end of said body adjacent said pressure responsive means.

4. A thermal actuator according to claim 3 wherein said heating means comprises variable heating means for generating heat along substantially the entire length of said body at an incrementally varying rate that increases progressively in an axial direction from the end of said body adjacent said pressure responsive means.

5. A thermal actuator according to claim I wherein said heat transfer means comprises heat dissipation means for dis sipating heat energy externally of said elongated body at an incrementally varying rate that increases progressively in an axial direction from the end of said longitudinal wall means adjacent said pressure responsive means.

6. A thermal actuator according to claim 1 wherein said longitudinal wall means is formed of electrically conductive material, and said heat transfer means further comprises electrical means adapted to provide electrical current flow through the length of said longitudinal wall means.

7. A thermal actuator according to claim 6 wherein said longitudinal wall means possesses an incrementally varying electrical resistance that decreases progressively in an axial direction from the end of said longitudinal wall means adjacent said pressure responsive means.

8. A thermal actuator according to claim 5 wherein said heat dissipation means comprises said longitudinal wall means which is adapted to dissipate heat externally of said elongated body at an incrementally varying rate that increases progressively in an axial direction from the end of said longitudinal wall means adjacent said pressure responsive means.

9. A thermal actuator according to claim 8 wherein said pressure responsive means is an elongated piston rod slidablc within said longitudinal wall means in response to volume changes in said elongated body and having an external surface in intimate contact with internal surface portions of said longitudinal wall means.

10. A thermal actuator according to claim 9 wherein said longitudinal wall means comprises electrically conductive material, and said heat transfer means further comprises electrical means adapted to provide current flow through the length of said longitudinal wall means.

11. A thermal actuator according to claim 1 including sensing means operatively coupled with said pressure responsive means and adapted to produce a signal indicative of the position of said pressure responsive means with respect to said longitudinal wall means.

12. A thermal actuator according to claim 2 including thermal detection means operatively coupled to said heating means and disposed in heat exchange relationship with said thermal actuator, said thermal detection means being adapted to deenergize said heating means in response to detection of a predetermined maximum temperature on said thermal actuator.

13. A thermal actuator according to claim 1 including limit means defining for said pressure responsive means a given maximum stroke and said elongated body comprises a volume that produces said maximum stroke before a substantial portion of said expansible material has been changed from a solid to a liquid state.

14. A thermal actuator according to claim 1 wherein said longitudinal wall means comprises a continuous hollow tube having one length portion wound into a coil with axially spaced turns and another straight length portion disposed within said coil and enclosing said pressure responsive means.

15. A thermal actuator according to claim 14 wherein said heat transfer means comprises heating means for producing heat transfer into said elongated body along substantially its entire length at an incrementally varying rate that decreases progressively in an axial direction from the end of said body adjacent said pressure responsive means.

16. A thermal actuator according to claim 15 wherein said heating means comprises variable heating mean for generating heat along substantially the entire length of said body at an incrementally varying rate that increases progressively in an axial direction from the end of said body adjacent said pressure responsive means.

17. A thermal actuator according to claim 14 wherein said longitudinal wall means comprise electrically conductive material and said heat transfer means further comprises electrical means adapted to provide electrical current flow through the entire length of said longitudinal wall means.

18. A thermal actuator according to claim 17 wherein said longitudinal wall means possesses an incrementally varying electrical resistance that decreases progressively in an axial direction from the end of said longitudinal wall means adjacent said pressure responsive means.

19. A thermal actuator according to claim 17 wherein said longitudinal wall means is adapted to dissipate heat energy externally of said elongated body at an incrementally varying rate that increases progressively in an axial direction from the end of said longitudinal wall means adjacent said pressure responsive means.

20. A thermal actuator according to claim 17 wherein the axial spacing between adjacent turns of said coil increases progressively along said coil from the end thereof joined to said straight length portion.

21. A thermal actuator according to claim 17 including a plurality of elongated alignment templets extending longitudinally of said coil and spaced about the circumference thereof, each said templet having a plurality of spaced openings each of which receive in close contact relationship one of said coil turns, and wherein the spacing between adjacent openings in each said templet increases progressively from the ends thereof adjacent said straight length portion to their opposite ends.

22. A thermal actuator according to claim 17 including an electrical transducer means operatively coupled to said pressure responsive means and responsive to movement thereof to produce an electrical signal indicative of the position of said pressure responsive means relative to said straight length tube portion.

23. A thermal actuator according to claim 22 including electrical control means operatively coupled to said electrical transducer means and adapted to regulate current flow to said longitudinal wall means in response to the signal output of said transducer means.

24. A thermal actuator according to claim 23 wherein said electrical transducer means comprises a hollow cylindrical transformer enclosed by said coiled tube portion and axially aligned with said straight tube portion.

25. A thermal actuator according to claim 24 therein said straight tube portion and said hollow cylindrical transformer are aligned in series, said pressure responsive means comprises first and second series connected piston sections, said first section is made of an electrically nonconductive material and normally disposed within said straight tube portion, and said second section is made of a ferromagnetic material and normally disposed within said hollow cylindrical transformer.

26. A thermal actuator according to claim 25 wherein said hollow cylindrical transformer comprises a hollow cylindrical primary winding and a hollow cylindrical secondary winding, said primary and secondary windings being concentrically disposed one within the other.

27. A thermal actuator according to claim 26 wherein said secondary winding comprises oppositely wound longitudinal sections.

28. A thermal actuator according to claim 26 including a magnetic shield means disposed between said coiled tube portion and said transformer means.

29. A thermal actuator according to claim 28 including a coiled spring member enclosing said tube coil and operatively connected to said piston means, said coiled spring member providing a biasing force that urges said piston means into said straight length tube portion.

30. A thermal actuator according to claim 29 including a cover means enclosing said coiled spring member. 

1. A thermal actuator comprising: a. an elongated body of expansible material, said expansible material being of a type that experiences substantial volume increase during changes between solid and liquid states and substantial volume decrease during changes between liquid and solid states, both changes in state being induced by changes in its temperature; b. pressure responsive means disposed adjacent one end of said elongated body and movable in response to volume changes therein; and c. heat transfer means in heat transfer relationship with said elongated body and adapted to produce incrementally varying temperatures longitudinally thereof; said heat transfer means comprising longitudinal wall means that restricts radial expansion of said elongated body.
 2. A thermal actuator according to claim 1 wherein said heat transfer means comprises heating means for producing longitudinally progressive melting of said elongated body with initial melting occurring at the end thereof adjacent said pressure responsive means.
 3. A thermal actuator according to claim 1 wherein said heat transfer means comprises heating means for producing heat transfer into said elongated body along substantially its entire length at an incrementally varying rate that decreases progressively in an axial direction from the end of said body adjacent said pressure responsive means.
 4. A thermal actuator according to claim 3 wherein said heating means comprises variable heating means for generating heat along substantially the entire length of said body at an incrementally varying rate that increases progressively in an axial direction from the end of said body adjacent said pressure responsive means.
 5. A thermal actuator according to claim 1 wherein said heat transfer means comprises heat dissipation means for dissipating heat energy externally of said elongated body at an incrementally varying rate that increases progressively in an axial direction from the end of said longitudinal wall means adjacent said pressure responsive means.
 6. A thermal actuator according to claim 1 wherein said longitudinal wall means is formed of electrically conductive material, and said heat transfer means further comprises electrical means adapted to provide electrical current flow through the length of said longitudinal wall means.
 7. A thermal actuator according to claim 6 wherein said longitudinal wall means possesses an incrementally varying electrical resistance that decreases progressively in an axial direction from the end of said longitudinal wall means adjacent said pressure responsive means.
 8. A thermal actuator according to claim 5 wherein said heat dissipation means comprises said longitudinal wall means which is adapted to dissipate heat externally of said elongated body at an incrementally varying rate that increases progressively in an axial direction from the end of said longitudinal wall means adjacent said pressure responsive means.
 9. A thermal actuator according tO claim 8 wherein said pressure responsive means is an elongated piston rod slidable within said longitudinal wall means in response to volume changes in said elongated body and having an external surface in intimate contact with internal surface portions of said longitudinal wall means.
 10. A thermal actuator according to claim 9 wherein said longitudinal wall means comprises electrically conductive material, and said heat transfer means further comprises electrical means adapted to provide current flow through the length of said longitudinal wall means.
 11. A thermal actuator according to claim 1 including sensing means operatively coupled with said pressure responsive means and adapted to produce a signal indicative of the position of said pressure responsive means with respect to said longitudinal wall means.
 12. A thermal actuator according to claim 2 including thermal detection means operatively coupled to said heating means and disposed in heat exchange relationship with said thermal actuator, said thermal detection means being adapted to deenergize said heating means in response to detection of a predetermined maximum temperature on said thermal actuator.
 13. A thermal actuator according to claim 1 including limit means defining for said pressure responsive means a given maximum stroke and said elongated body comprises a volume that produces said maximum stroke before a substantial portion of said expansible material has been changed from a solid to a liquid state.
 14. A thermal actuator according to claim 1 wherein said longitudinal wall means comprises a continuous hollow tube having one length portion wound into a coil with axially spaced turns and another straight length portion disposed within said coil and enclosing said pressure responsive means.
 15. A thermal actuator according to claim 14 wherein said heat transfer means comprises heating means for producing heat transfer into said elongated body along substantially its entire length at an incrementally varying rate that decreases progressively in an axial direction from the end of said body adjacent said pressure responsive means.
 16. A thermal actuator according to claim 15 wherein said heating means comprises variable heating mean for generating heat along substantially the entire length of said body at an incrementally varying rate that increases progressively in an axial direction from the end of said body adjacent said pressure responsive means.
 17. A thermal actuator according to claim 14 wherein said longitudinal wall means comprise electrically conductive material and said heat transfer means further comprises electrical means adapted to provide electrical current flow through the entire length of said longitudinal wall means.
 18. A thermal actuator according to claim 17 wherein said longitudinal wall means possesses an incrementally varying electrical resistance that decreases progressively in an axial direction from the end of said longitudinal wall means adjacent said pressure responsive means.
 19. A thermal actuator according to claim 17 wherein said longitudinal wall means is adapted to dissipate heat energy externally of said elongated body at an incrementally varying rate that increases progressively in an axial direction from the end of said longitudinal wall means adjacent said pressure responsive means.
 20. A thermal actuator according to claim 17 wherein the axial spacing between adjacent turns of said coil increases progressively along said coil from the end thereof joined to said straight length portion.
 21. A thermal actuator according to claim 17 including a plurality of elongated alignment templets extending longitudinally of said coil and spaced about the circumference thereof, each said templet having a plurality of spaced openings each of which receive in close contact relationship one of said coil turns, and wherein the spacing between adjacent openings in each said templet increases progressively from the ends thereof adjacent saId straight length portion to their opposite ends.
 22. A thermal actuator according to claim 17 including an electrical transducer means operatively coupled to said pressure responsive means and responsive to movement thereof to produce an electrical signal indicative of the position of said pressure responsive means relative to said straight length tube portion.
 23. A thermal actuator according to claim 22 including electrical control means operatively coupled to said electrical transducer means and adapted to regulate current flow to said longitudinal wall means in response to the signal output of said transducer means.
 24. A thermal actuator according to claim 23 wherein said electrical transducer means comprises a hollow cylindrical transformer enclosed by said coiled tube portion and axially aligned with said straight tube portion.
 25. A thermal actuator according to claim 24 therein said straight tube portion and said hollow cylindrical transformer are aligned in series, said pressure responsive means comprises first and second series connected piston sections, said first section is made of an electrically nonconductive material and normally disposed within said straight tube portion, and said second section is made of a ferromagnetic material and normally disposed within said hollow cylindrical transformer.
 26. A thermal actuator according to claim 25 wherein said hollow cylindrical transformer comprises a hollow cylindrical primary winding and a hollow cylindrical secondary winding, said primary and secondary windings being concentrically disposed one within the other.
 27. A thermal actuator according to claim 26 wherein said secondary winding comprises oppositely wound longitudinal sections.
 28. A thermal actuator according to claim 26 including a magnetic shield means disposed between said coiled tube portion and said transformer means.
 29. A thermal actuator according to claim 28 including a coiled spring member enclosing said tube coil and operatively connected to said piston means, said coiled spring member providing a biasing force that urges said piston means into said straight length tube portion.
 30. A thermal actuator according to claim 29 including a cover means enclosing said coiled spring member. 